The present invention relates to a magnetic brush apparatus for an electrostatic printing system and, more particularly, to a magnetic brush apparatus for forming a magnetic brush by a developer called one-component developer.
Two types of the magnetic brush apparatuses are known. One of them is of the type to form a magnetic brush by using a developer mixture called a two component developer, which is composed of powdered developer or toner and magnetic carrier such as powdered iron. The other is of the type to form the same by the powdered developer called a one-component developer, which is composed of powdered toner containing magnetic particles covered with resin which is colored by agent. The magnetic brush apparatus of this kind comprises a hopper which containes the developer and supplies the developer, a tubular sleeve which is made of nonmagnetic material and is supplied at the surface with developer, a magnetic roller which produces a magnetic field on the sleeve surface to form a magnetic brush and a doctor blade for defining the height of a magnetic brush formed on the sleeve surface to be at a predetermined level. With rotation of either the magnetic roller or the sleeve, the magnetic brush of a fixed heigh in transfered and is contacted with a photosensitive layer of photoconductive paper on which an electrostatic latent charge image is formed.
The magnetic brush apparatus of the one-component type and the two-component type, which are about the same in the construction, suffer from the following disadvantages. In the two-component type magnetic brush apparatus, the life time of the developer is relatively short so that the worn developer must be replaced by a new one at which a given number of sheet is copied. On the other hand, the one-component type magnetic brush apparatus has not such a disadvantage but has the following disadvantages. In this type apparatus, the magnetic brush formed is thin and uneven, compared to the two-component type apparatus. For this, the distance between the sleeve and photoconductive paper must be selected to be narrow and an amount of the developer supplied must always be controlled optimum. Therefore, the doctor blade in the magnetic brush apparatus using the one-component developer is more important than that in the magnetic brush apparatus. In the two-component developer, the size of each toner particle is 3.about.20 .mu..phi. and the size of each carrier particle is 50.about.150 .mu..phi.. The magnetic brush may be formed by relatively large particles and further the distance between the tip of the doctor blade and the sleeve is usually relatively long, e.g. 3.about.5 mm. Therefore, mere passing of the magnetic brush through a space between the doctor blade and the sleeve can provide a given height of the magnetic brush.
On the other hand, in the one-component developer the size of each toner particle is relatively small, for example, 5.about.25.mu.. The magnetic brush is formed by relatively small particles. The distance between the tip of the doctor blade and the sleeve must be precisely selected to be 0.3 to 0.5 mm. If the magnetic brush properly passes through a space between the doctor blade and the sleeve, that is to say, the magnetic brush is properly scraped off the magnetic brush must be controlled to have a proper height. In fact, the space is narrower and the developer particle is small in diameter and further the developer is of one-component type. Because of this, the transferring developer particles colligate to each other becoming solidified and staying at the solidified location. As a result, the space between the doctor blade and the sleeve is closed. Further, the height of the magnetic brush formed is uneven or the amount of the transferring developer particles unstably varies. In the case of the one-component type apparatus, it is very difficult to obtain a uniform level of the magnetic brush height by scraping off the magnetic brush formed.